1. Field of the Invention
The present invention relates to methods and compositions for preventing or alleviating the loss of drilling fluids into a subterranean formation during drilling of boreholes in said formation. More particularly, the present invention relates to methods and compositions suitable for preventing or alleviating lost circulation in a producing zone.
2. Description of Relevant Art
In the oil and gas industry, a common problem in drilling wells or boreholes in subterranean formations is the loss of circulation of fluids, such as drilling fluids or muds or well treatment fluids, in a well or borehole during the drilling. Such lost fluids typically go into fractures induced by excessive mud pressures, into pre-existing open fractures, or into large openings with structural strength in the formation.
A large variety of materials have been used or proposed in attempts to cure lost circulation. Generally, or commonly, such materials may be fibrous (like shredded automobile tires or sawdust), flaky (such as wood chips or mica flakes), granular (such as ground nutshells), or slurries, whose strength increases with time after placement, (such as hydraulic cement, or polyacrylamide dispersed in water, emulsified, and often crosslinked in a paraffinic mineral oil, often also containing bentonite). However, such lost circulation compositions have often been unsuccessful due to overly delayed and inadequate viscosity development. The delay in developing viscosity allows the lost circulation or sealing composition to be diluted and displaced into subterranean producing zone(s) into or near the lost circulation zone thereby damaging the producing zone(s). Also, such sealing compositions have often been difficult or impossible to remove from the subterranean producing zones into which they have penetrated.
To prevent damage to a producing zone in or near a lost circulation zone, the producing zone should be sealed with a sealing composition that can subsequently be removed to prevent drilling fluid damage to the producing zone. There continues to be a need for improved methods and compositions to effect such sealing of producing zones where the sealing material can be readily and completely or substantially completely removed from the zone and where the sealing material does not otherwise damage the producing zone.